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Highly compressed ammonia forms an ionic crystal


Ammonia is an important compound with many uses, such as in the manufacture of fertilizers, explosives and pharmaceuticals. As an archetypal hydrogen-bonded system, the properties of ammonia under pressure are of fundamental interest, and compressed ammonia has a significant role in planetary physics. We predict new high-pressure crystalline phases of ammonia (NH3) through a computational search based on first-principles density-functional-theory calculations1. Ammonia is known to form hydrogen-bonded solids2,3,4,5,6, but we predict that at higher pressures it will form ammonium amide ionic solids consisting of alternate layers of NH4+ and NH2 ions. These ionic phases are predicted to be stable over a wide range of pressures readily obtainable in laboratory experiments. The occurrence of ionic phases is rationalized in terms of the relative ease of forming ammonium and amide ions from ammonia molecules, and the volume reduction on doing so. We also predict that the ionic bonding cannot be sustained under extreme compression and that, at pressures beyond the reach of current static-loading experiments, ammonia will return to hydrogen-bonded structures consisting of neutral NH3 molecules.

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Figure 1: Enthalpies per NH3 unit of various phases as a function of pressure.
Figure 2: The P 21/c (left) and P 212121 (right) molecular structures at 20 GPa.
Figure 3: The P m a2, P 21/m and P n m a structures.


  1. Martin, R. M. Electronic Structure: Basic Theory and Practical Methods (Cambridge University Press, Cambridge, 2004).

    Book  Google Scholar 

  2. Reed, J. W. & Harris, P. M. Neutron diffraction study of solid deuteroammonia. J. Chem. Phys. 35, 1730–1737 (1961).

    CAS  Article  Google Scholar 

  3. Hewat, A. W. & Riekel, C. The crystal structure of deuteroammonia between 2 and 180 K by neutron powder profile refinement. Acta Crystallogr. A A35, 569–571 (1979).

    CAS  Article  Google Scholar 

  4. Leclerq, F., Damay, P. & Foukani, M. Structure of powder deuteroammonia between 2 and 180 K revisited: A refinement of the neutron diffraction pattern taking into account molecular reorientations: analysis of the diffuse intensity. J. Chem. Phys. 102, 4400–4408 (1995).

    Article  Google Scholar 

  5. Loveday, J. S. et al. Structure of deuterated ammonia IV. Phys. Rev. Lett. 76, 74–77 (1996).

    CAS  Article  Google Scholar 

  6. Datchi, F. et al. Solid ammonia at high pressure: A single-crystal x-ray diffraction study to 123 GPa. Phys. Rev. B 73, 174111 (2006).

    Article  Google Scholar 

  7. Nelson, D. D. Jr., Fraser, G. T. & Klemperer, W. Does ammonia hydrogen bond? Science 238, 1670–1674 (1987).

    CAS  Article  Google Scholar 

  8. Hubbard, W. B. Interiors of the giant planets. Science 214, 145–149 (1981).

    CAS  Article  Google Scholar 

  9. Sasselov, D. D. Extrasolar planets. Nature 451, 29–31 (2008).

    CAS  Article  Google Scholar 

  10. Cavazzoni, C. et al. Superionic and metallic states of water and ammonia at giant planet conditions. Science 283, 44–46 (1999).

    CAS  Article  Google Scholar 

  11. Gauthier, M., Pruzan, Ph., Chervin, J. C. & Besson, J. M. Raman scattering study of ammonia up to 75 GPa: Evidence for bond symmetrization at 60 GPa. Phys. Rev. B 37, 2102–2115 (1988).

    CAS  Article  Google Scholar 

  12. Ninet, S., Datchi, F., Saitta, A. M., Lazzeri, M. & Canny, B. Raman spectrum of ammonia IV. Phys. Rev. B 74, 104101 (2006).

    Article  Google Scholar 

  13. Gauthier, M., Pruzan, Ph., Chervin, J. C. & Polian, A. Brillouin study of liquid and solid ammonia up to 20 GPa. Solid State Commun. 68, 149–153 (1988).

    CAS  Article  Google Scholar 

  14. Sakashita, M., Yamawaki, H., Fujihisa, H. & Aoki, K. Phase study of NH3 to 100 GPa by infrared absorption. Rev. High Pressure Sci. Technol. 7, 796–798 (1998).

    CAS  Article  Google Scholar 

  15. Kamb, B. & Davis, B. L. Ice VII, the densest form of ice. Proc. Natl Acad. Sci. USA 52, 1433–1439 (1964).

    CAS  Article  Google Scholar 

  16. Struzhkin, V. V., Goncharov, A. F., Hemley, R. J. & Mao, H.-K. Cascading Fermi resonances and the soft mode in dense ice. Phys. Rev. Lett. 78, 4446–4449 (1997).

    CAS  Article  Google Scholar 

  17. Goncharov, A. F., Struzhkin, V. V., Mao, H.-K. & Hemley, R. J. Raman spectroscopy of dense H2O and the transition to symmetric hydrogen bonds. Phys. Rev. Lett. 83, 1998–2001 (1999).

    CAS  Article  Google Scholar 

  18. Aoki, K., Yamawaki, H. & Sakashita, M. Observation of Fano interference in high-pressure ice VII. Phys. Rev. Lett. 76, 784–786 (1996).

    CAS  Article  Google Scholar 

  19. Fortes, A. D., Brodholt, J. P., Wood, I. G. & Voc˘adlo, L. Hydrogen bonding in solid ammonia from ab initio calculations. J. Chem. Phys. 118, 5987–5994 (2003).

    CAS  Article  Google Scholar 

  20. Pickard, C. J. & Needs, R. J. High pressure phases of silane. Phys. Rev. Lett. 97, 045504 (2006).

    Article  Google Scholar 

  21. Pickard, C. J. & Needs, R. J. Structure of phase III of hydrogen. Nature Phys. 3, 473–476 (2007).

    CAS  Article  Google Scholar 

  22. Pickard, C. J. & Needs, R. J. Metallization of aluminum hydride at high pressures: A first-principles study. Phys. Rev. B 76, 144114 (2007).

    Article  Google Scholar 

  23. Pickard, C. J. & Needs, R. J. When is H2O not water? J. Chem. Phys. 127, 244503 (2007).

    Article  Google Scholar 

  24. Eremets, M. I., Trojan, I. A., Medvedev, S. A., Tse, J. S. & Yao, Y. Superconductivity in hydrogen dominant materials: silane. Science 319, 1506–1509 (2008).

    CAS  Article  Google Scholar 

  25. Goncharenko, I. et al. Pressure-induced hydrogen-dominant metallic state in aluminum hydride. Phys. Rev. Lett. 100, 045504 (2008).

    Article  Google Scholar 

  26. Segall, M. D., Shah, R., Pickard, C. J. & Payne, M. C. Population analysis of plane-wave electronic structure calculations of bulk materials. Phys. Rev. B 54, 16317–16320 (1996).

    CAS  Article  Google Scholar 

  27. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).

    CAS  Article  Google Scholar 

  28. Liebman, J. F. Existence and estimated enthalpies of formation of ammonium hydroxide, hydronium amide, and some related species. Struct. Chem. 8, 313–315 (1997).

    CAS  Article  Google Scholar 

  29. Somayazulu, M. et al. Novel broken symmetry phase from N2O at high pressures and high temperatures. Phys. Rev. Lett. 87, 135504 (2001).

    CAS  Article  Google Scholar 

  30. Meng, Y. et al. Hard x-ray radiation induced dissociation of N2 and O2 molecules and the formation of ionic nitrogen oxide phases under pressure. Phys. Rev. B 74, 214107 (2006).

    Article  Google Scholar 

  31. Fortes, A. D., Brodholt, J. P., Wood, I. G., Voc˘adlo, L. & Jenkins, H. D. B. Ab initio simulation of ammonia monohydrate (NH3·H2O) and ammonium hydroxide (NH4OH). J. Chem. Phys. 115, 7006–7014 (2001).

    CAS  Article  Google Scholar 

  32. Clark, S. J. et al. First principles methods using CASTEP. Z. Kristallogr. 220, 567–570 (2005).

    CAS  Google Scholar 

  33. Vanderbilt, D. Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 41, 7892–7895 (1990).

    CAS  Article  Google Scholar 

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We thank S. Clark for help with the Raman calculations. R.J.N. was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the UK.

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Correspondence to Chris J. Pickard.

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C.J.P. is an author of the CASTEP code, used in this work and sold commercially by Accelrys.

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Pickard, C., Needs, R. Highly compressed ammonia forms an ionic crystal. Nature Mater 7, 775–779 (2008).

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